This chapter is from the book

This chapter is from the book

You've gotten the ball rolling on the site and have shown that
you've got what it takes to get the job donenow your boss wants you
to wow him. He wants to make the site more dynamic and give it a little bit of
"flash." He has checked out the competition's sites, and they all
have image rollovers on theirsJavaScript rollovers are used to swap an
image with a different version of that image when the user moves the cursor over
it. Conversely, when the user moves the cursor off of the image, the new image
is replaced with the original. This technique is widely used on Web pages, and
your boss feels this is just what your site needs to spruce it up.

The first thing to do is choose which graphics we want to be affected by the
rollovers. The best place for image rollovers on most pages is on the main
navigation images, and the Shelley Biotech site is no different (see Figure
21).

One of the designers has come up with a great graphic treatment for the
rollovers (see Figure 22), and it's now up to you to implement them.
There are three parts to creating an image rollover: Define the IMAGE
objects, create the functions that will do the work, and insert the necessary
JavaScript event handlers into your image and anchor tags. The first step in
writing our script is the creation of the IMAGE objects.

Project I: Image Rollover Script

Creating and Inserting the Image Objects

Before we get to the creation of the image objects themselves, we must decide
where to put our script. Because the rollovers will be on every page of the
site, it makes sense to place our script into an external JavaScript file. That
way, we only need to place it in a single location instead of on each page. We
have already created an external JavaScript file called jsFunctions.js
for one of our previous projects, so we can simply add our new script to that
file. The home page already has the call to the external file, so we can just
copy that <script> tag to all of the other pages on the site.

The first step in our script is to cloak for older browsers that don't
support the IMAGE object, which is necessary for our rollover script.
In Chapter 1 we learned how to use JavaScript to gather the browser and platform
information. We could use a variation of that method in which we set up a bunch
of if statements that test for every browser that doesn't support
the IMAGE object; however, this would be loads of work. Fortunately for
us, there is a simpler way. We can test if the user's browser supports
rollovers using only a single if statement, as shown in the following
lines of code:

// Creation of the image objects
if (document.images) {
. . .
}

In the preceding lines of code we used

document.images

as the condition in our if statement. The condition returns a value
of true if the browser supports the IMAGE object and a value
of false if it does not. By inserting our code within this if
statement, we are in effect cloaking our code from browsers that cannot handle
the script.

When an HTML page is loaded and the browser creates the objects that make up
the JavaScript hierarchy, every image laid out on the page is made into an
object and put into an array called images. This is the first time we
have come across arrays, so let's take a quick look at what an array is
and, in particular, the images array and how we are going to use it.

You can think of an array as a filing cabinet. Let's say you have a page
with four images on it. When the browser reads the HTML file, it goes down the
page, and when it reaches the first image, it creates an IMAGE object
for it and stores it in the first drawer of our filing cabinet. When it reaches
the second image, it again creates a new IMAGE object, and then puts it
into the second drawer of the filing cabinet, and so on until each of the images
on the page has its own IMAGE object, which is stored in its own
drawer.

When the browser needs to reference any of those images, it knows in which
drawer to look for each of them. You, too, can reference the array and the
IMAGE objects held thereinthere are two ways to do this. The
first is to reference the location in the array at which the IMAGE
object resides. The problem with this method is that if you add an image
somewhere on the page, the position of all of the images below it will change.
This will cause you to go through your whole script and make sure an image you
were calling isn't now in a different position. The second way, which we
use in this script, takes care of this problem.

If you specify a NAME attribute in your <img> tags,
you can then reference that graphic in the array by the name that you have
assigned to it. With this method, even if you add other graphics to the page,
you will still be able to reference the IMAGE object the same way, and
your script won't be adversely affected.

There are six navigation images that will be affected by our rollover script,
so for each of those images, we need to assign a NAME attribute in the
<img> tags.

In the preceding lines of HTML, we added a NAME attribute to each of
the six <img> tags. The name for each is the first word of the
category that the graphic represents. JavaScript is case-sensitive, so take
notice that the first letter of each name is capitalized. Now that we have the
names of these images squared away, we can move on to the creation of some new
IMAGE objects that we will need for our script.

Because the images that we want to come up when the user rolls over one of
the six graphics aren't explicitly placed on the page with HTML, we need to
create two new IMAGE objects for each image we want to roll over. This
will add these images to the images array and allow us to access their
properties. Let's start by creating the IMAGE objects for the
Products image.

In this first line, we initialize a new IMAGE object by using an
image constructor, the syntax for which is

New Image(width, height);

By putting the image constructor on the right-hand side of the assignment
operator, and the name that we wish the IMAGE object to have on the
left, we have just created a new IMAGE object called
ProductsOn.

The naming of these new objects is very important to the operation of the
script. The first part of the name should be the same as the name of the image
to which it corresponds that is already on the page. For example, in this first
line, we created an object to hold the location of the rolled over version of
the Products graphic, so the name starts with Products.
Because this object holds the rolled over version of the graphic, the second
part of the name is On. When combined, we have a new object with the
name ProductsOn. For each of the six navigation images, we need not
only an object to hold the rolled over version of the graphic but one to hold a
regular version of the graphic as well. This second object's name for our
Products image will also start with Products but will end with
Off. Let's add this new IMAGE object after our first
one.

If you deviate from this naming convention, the functions that we will be
writing won't know which IMAGE object to use for replacement, and
you will get errors. We now have the two new IMAGE objects that we will
need for the Products rollover, but at the moment they are empty. We
need to assign actual images to them. We do this by assigning the location (URL)
of the image to the source property of the IMAGE object, as
follows:

We have completed On and Off objects for the
Products rollover, but we now have to create objects for each of the
other five image rollovers. We do this using the same naming conventions that we
used for our first objects.

Creating the IMAGE objects in this manner also has another desired
effect. When the browser creates the new IMAGE objects, it loads the
images into its cache, thereby preloading it into memory. This preloading allows
the image to display immediately upon rollover. It is important to keep this
preloading in mind when creating your HTML pagesif you do not put
Height, Width, and alt attributes into your
<img> tags, some browsers will wait to display any of the page
until all of the content and images are loaded into memory. You can run into
some serious load times if you have a lot of IMAGE objects loading for
rollovers in the background, so make sure you put all of the necessary tags into
your HTML.

We have now created IMAGE objects for all of the graphics that we
need to accomplish our rollovers. Our next step is the creation of the functions
that will run our rollovers.

Image Rollover Functions

When the user rolls over one of our graphics, JavaScript event handlers call
the functions we are going to write to actually perform the image replacements.
Functions are an essential part of JavaScript; a function is a set of JavaScript
statements that perform specific tasks. We cover both defining and calling
functions in this example, but for now, let's concentrate on defining the
functions we need for our rollovers. To define a function, you need four basic
elements: the function keyword, a name for the function, a set of
arguments that are separated with commas, and the statements that you want the
function to execute.

We need to create two functions for our rollovers to work: one that switches
to the On graphic when we roll onto an image and another to switch back
to the Off graphic when we roll off the image. Generally, you should
define the functions for a page in the <head> of your HTML, so
let's start our functions right after our IMAGE objects.

// function to turn on rolled over graphic
function on(pic) {
. . .
}

The first step in defining a function is to call the function keyword
followed by the name of our function; in this case, we'll call it
On. Following the name, within its set of parentheses, we need to put a
list of variables separated by commas, one for each argument we want to pass
into the function. These variables will let us pass values into our function
from the event handlers that call the function. We need to pass only a single
argument into our function; we'll use the variable pic to hold
that argument.

Next, we need to insert the set of instructions that we want the function to
executethese statements are enclosed within curly brackets.

The first of the statements that we want the function to execute is an
if statement that will check for older browsers. If the browser
supports the IMAGE object, the next line will be executed. In this
line, we tell the browser to replace the source of the image that is being
rolled over with the source of the On version of the graphic. A lot is
happening in this line, so let's break it down into smaller parts. First,
let's look at the left-hand side of the assignment operator.

document.images[pic].src

We reference the source property of the image located at the position in the
images array that has the value of the variable pic, so
let's say the value of pic is Products. We would be
referencing the source property of the ProductsIMAGE object
and assigning it the value passed to it from the right-hand side of the
assignment operator.

If we explicitly reference the images, like so:

document.product.src

we need separate functions for every image we wanted a rollover fornot
the most efficient way to code. When scripting, you always should think about
how you can most efficiently write your code. Any time you can use variables in
this way, it will save you a lot of time and trouble. There are, however,
occasions when you will want to call a specific image every time the function is
called, and the preceding line of code is an example of how to do that.

On the right side of the operator, we use the eval() statement to
combine the value of pic with a string to create a new value to assign
as the source of the image being rolled over.

eval(pic + "On.src");

If the value of pic is Products, the eval()
statement will return ProductsOn.src, and the value of the source of
the ProductsOnIMAGE object will be assigned to the source of
the Products IMAGE object. When this happens, the image on the page
will change to the On image until the user rolls off.

When the user rolls off the image, another function will be called. This is
the second function that we have to createlet's call this function
Off.

This function should look similar to your On function; in fact, the
only difference is that the second half of what is being evaluated on the
right-hand side of the assignment operator is Off.src instead of
On.src. Therefore, when this line is evaluated, it will assign the
source of the ProductsOffIMAGE object to be the source of the
ProductsIMAGE object, thereby returning the image to its
regular look.

Our functions are now complete and we are almost finished with the script.
All that's left for us to do is to put the proper JavaScript event handlers
into our HTML code.

Creating and Inserting the Event Handlers

You will find that most things in JavaScript are event-drivenevents
generally are the result of some action on the part of the user. If a user rolls
over a link or changes the value of a text field, these would constitute events.
To make use of these events with your JavaScript, you must define an event
handler to react to these events. There are many predefined event handlers that
you can use for this purpose; the two that we use for our rollovers are
onMouseOver and onMouseOut. Let's first see what our
existing HTML looks like without the event handlers.

When an event handler is called, it executes any JavaScript statements that
follow the handler. We enclose these statements in quotes and separate them with
semicolons. The first thing we want our event handler to do is call our
On function. To call a function, you simply call its name and enclose
any arguments you wish to pass along to the function in parentheses separated by
commas.

on('Products');

The preceding statement calls the On function and passes it the
value Products; again, we are passing it Products because that
is the name of the image that we want our function to affect.

Another often seen addition to rollovers is the display of a relevant phrase
in the browser's status bar. To do this, we add another statement to our
event handler.

By calling on the WINDOW object's status property, we
can display text in the browser's status bar by assigning it a new value.
The last addition that we need to make to our event handler is the following
statement:

return true;

This tells the JavaScript engine that this is the end of what needs to be
executed in the event handler and it can go on with the rest of its business.

Well, one event handler down, and one more to go. Let's add our
onMouseOut handler to take care of things when the user rolls off an
image.

For our onMouseOut handler, we call the Off function, again
passing it the name of the graphic that we want to affect. To turn off the
phrase that we put into the status bar, we now reset the window.status
to a blank value and then leave the handler with the return
statement.

Now that we have inserted the event handlers for the Products image,
we need to put some in for the rest of the images we want to have rollovers for.
The syntax will be the same, the only difference being that you have to change
the value that you are passing to the functions to the name of the graphic that
you want to be changed.

Here are the steps we took to insert the event handlers into the HTML:

We added the onMouseOver event handlers to each of the six
images' <a href> tags. Within that handler we called the
On function, passing it the name of the graphic that we were rolling
over, changed the status bar to display a new message, and added the
return command to exit the handler.

We added the onMouseOut event handlers to each of the six
images' <a href> tags. Within that handler we called the
Off function, passing it the name of the graphic that we were rolling
over, reset the status bar to nothing, and added the return command to
exit the handler.

We introduced several new aspects of JavaScript in this first script,
including

A new method of cloaking that looks for browsers that support specific
objects that are needed in your script.

An introduction to arrays, specifically the images
array.

How to create new IMAGE objects and how to change their source
property.

How to create a function.

The concept of event handlers and how to use two of them:
onMouseOver and onMouseOut.

How to change the message being displayed in the status bar of the
browser window.